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Study on the effect of melamine on the growth of Streptococcus thermophilus

Streptococcus thermophilus is a key lactic acid bacterium widely used in yogurt and fermented dairy production, contributing to texture, flavor, and nutritional value. However, concerns about melamine contamination in raw milk have raised questions: Can melamine affect the growth and fermentation performance of Streptococcus thermophilus? Can the bacterium degrade melamine to reduce its content in dairy products?

This article details the study on the effect of melamine on the growth of streptococcus thermophilus.

Key Background: Melamine in Dairy & Streptococcus thermophilus

1. Melamine powder Contamination Risks

Melamine powder (C₃H₆N₆) is an industrial chemical with a nitrogen content of 66.7%—4x higher than protein. Illegally added to milk to inflate protein content, it poses severe health risks (e.g., kidney stones). Global food safety standards set limits:
  • Infant formula: ≤1 mg/kg
  • Liquid milk, regular milk powder: ≤2.5 mg/kg
  • Foods with ≥15% milk content: ≤2.5 mg/kg
Trace melamine powder may also enter milk via packaging materials or food chain enrichment, making its interaction with fermentation bacteria a critical research focus.

2. Role of Streptococcus thermophilus in Fermentation

Streptococcus thermophilus is a facultative anaerobic Gram-positive bacterium that:
  • Converts lactose to lactic acid, lowering pH and coagulating milk proteins to form yogurt.
  • Produces flavor compounds (e.g., diacetyl) and bioactive peptides.
  • Relies on organic nitrogen sources (e.g., peptone, beef extract) for growth—unable to utilize inorganic nitrogen.
Understanding how melamine interacts with this bacterium is essential to ensuring the quality and safety of fermented dairy products.

Experimental Design: Testing Melamine’s Impact

The study used MRS liquid medium (a standard for lactic acid bacteria culture) with varying melamine concentrations to evaluate three key aspects:
  • Whether Streptococcus thermophilus can use melamine as a nitrogen source.
  • Melamine’s effect on the bacterium’s growth and acid production.
  • Changes in colony count, morphology, and growth cycle.

Experimental Setup

Materials & Methods
  • Bacteria: Streptococcus thermophilus (activated in skim milk medium).
  • Media:
    1. Control groups: Standard MRS medium (complete nitrogen source) and nitrogen-free MRS medium.
    2. Experimental groups: MRS medium with melamine concentrations of 0.25‰, 0.5‰, 1‰, 2‰, 3‰, and saturated (4‰).
  • Culture conditions: 42℃ for 48h (optimal temperature for Streptococcus thermophilus).
  • Measurements: pH changes (indicative of acid production/growth), colony count (dilution plate method), and microscopic observation of colony morphology.

Core Findings: Melamine’s Minimal Impact on Streptococcus thermophilus

1. Melamine powder Cannot Be Used as a Nitrogen Source

Streptococcus thermophilus requires organic nitrogen (e.g., peptone, yeast extract) for growth. The study confirms:
  • In standard MRS medium (complete nitrogen), pH dropped to 3.20 after 48h—indicating vigorous growth and acid production.
  • In a nitrogen-free medium, pH remained high (5.16), and growth was severely inhibited.
  • In media with melamine as the sole nitrogen source, pH increased slightly with melamine concentration (5.30–5.88)—no acid production or growth.
Conclusion: Melamine’s nitrogen is unavailable to Streptococcus thermophilus, which lacks the enzymes to degrade its triazine ring structure.

2. Weak Inhibitory Effect on Growth & Acid Production

Within water-soluble concentrations (0–4‰), melamine has a mild, concentration-dependent inhibitory effect:
  • pH changes: After 24h, the control group (no melamine) had a pH of 3.36, while the saturated melamine group had a pH of 3.58 (slightly higher, indicating less acid production).
  • Growth cycle: Melamine extends the lag phase (3–6h) of Streptococcus thermophilus, but the logarithmic and stationary phases are unaffected. By 24h, all groups reach similar growth levels.
  • Colony count: Colony numbers decrease by <200 CFU/mL with increasing melamine concentration—negligible compared to the total count (≈10⁷ CFU/mL).

3. No Significant Change in Colony Morphology

  • Colony shape remains consistent across all groups (round, smooth, white).
  • Colony size decreases slightly at saturated melamine concentration, but this does not affect fermentation functionality.

Practical Implications for the Dairy Industry & Food Safety

1. Yogurt Fermentation Is Not Compromised

The weak inhibitory effect of melamine (even at saturated concentration) does not hinder yogurt production:
  • Streptococcus thermophilus still produces sufficient lactic acid to coagulate milk.
  • Fermentation time and final product texture/flavor are unchanged.
  • Trace melamine (within legal limits) in raw milk will not affect industrial yogurt fermentation.

2. Streptococcus thermophilus cannot reduce the melamine content

Since the bacterium cannot degrade or utilize melamine, fermented dairy products (e.g., yogurt) will retain the same melamine concentration as the raw milk. Manufacturers must:
  • Source raw milk with melamine levels within legal limits.
  • Implement strict quality control (e.g., HPLC/GC-MS testing) for raw materials.

3. Consumer Safety Notes

  • Yogurt and fermented dairy cannot “detoxify” melamine-contaminated milk.
  • Legal melamine levels (≤2.5 mg/kg) in raw milk are safe for fermentation and consumption.
  • Infants and vulnerable groups should consume products meeting stricter limits (≤1 mg/kg for formula).

FAQ

Q1: Will melamine-contaminated milk fail to ferment into yogurt?
A1: No. Even at saturated melamine concentration (4‰), the inhibitory effect is too weak to prevent fermentation. Yogurt production will proceed normally.

Q2: Can other lactic acid bacteria (e.g., Lactobacillus bulgaricus) degrade melamine?
A2: Current research suggests no. Most lactic acid bacteria rely on organic nitrogen and lack the metabolic pathways to break down melamine’s triazine ring.

Q3: Does yogurt’s acidity reduce melamine toxicity?
A3: No. Melamine is stable in acidic environments (yogurt pH ≈3.5) and does not decompose or become less toxic. Toxicity depends on the ingestion dose, not pH.

Q4: How to ensure fermented dairy is free of excessive melamine?
A4: Manufacturers must test raw milk using standardized methods (e.g., GB/T 22388-2008 in China). Consumers should choose reputable brands with transparent quality control.

conclusion

Melamine has minimal impact on Streptococcus thermophilus: it cannot be used as a nitrogen source, nor does it significantly inhibit yogurt fermentation. However, the bacterium cannot degrade melamine, so raw milk quality remains the key to ensuring the safety of fermented dairy products. For manufacturers, strict raw material testing and compliance with safety standards are essential. For consumers, understanding this interaction helps alleviate concerns about yogurt fermentation safety while emphasizing the importance of choosing trusted products.

As food safety regulations evolve, ongoing research into melamine’s interaction with food microorganisms will further enhance our ability to mitigate risks and protect public health.

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